This section is intended to introduce the reader to various aspects of the art that may be related to various aspects of the present invention. The following discussion is intended to provide information to facilitate a better understanding of the present invention. Accordingly, it should be understood that statements in the following discussion are to be read in this light, and not as admissions of prior art.
Multi-antenna techniques can significantly increase the data rates and reliability of a wireless communication system. The performance is in particular improved if both the transmitter and the receiver are equipped with multiple antennas. This results in a multiple-input multiple-output (MIMO) communication channel and such systems and/or related techniques are commonly referred to as MIMO.
The LTE standard is currently under development. A core component in LTE is the support of MIMO antenna deployments and MIMO related techniques. A current working assumption in LTE is the support of a spatial multiplexing mode with possibly channel dependent precoding. The spatial multiplexing mode is aimed for high data rates in favorable channel conditions. An illustration of the spatial multiplexing mode is provided in FIG. 1.
As seen, the information carrying symbol vectors s is multiplied by an NT×r precoder matrix WNT×r. The matrix is often chosen to match the characteristics of the NR×NT MIMO channel H. The r symbols in s each correspond to a layer and r is referred to as the transmission rank. LTE uses OFDM and hence the received NR×1 vector yk for a certain resource element on subcarrier k (or alternatively data resource element number k), assuming no inter-cell interference, is thus modeled byyk=HWNT×rsk+ek  (1)where ek is a noise vector obtained as realizations of a random process.
The UE may, based on channel measurements in the forward link, transmit recommendations to the base station of a suitable precoder to use. A single precoder that is supposed to cover a large bandwidth (wideband precoding) may be fed back. It may also be beneficial to match the frequency variations of the channel and instead feedback a frequency-selective precoding report, e.g. several precoders, one per subband.
Channel dependent precoding as above typically requires substantial signaling support, particularly for frequency-selective precoding. Not only is feedback signaling in the reverse link, as mentioned previously, needed, but typically also signaling in the forward link is required to indicate which precoder was actually used in the forward link transmission since the forward link transmitter might not be certain that it obtained a correct precoder report from the (forward link) receiver.
One way of reducing the signaling overhead in the forward link is to signal some kind of precoder confirmation, e.g., whether the transmitter used the same precoders as fed back by the receiver or not. A single bit could be used for this purpose; a value of 1 could mean that the transmitter follows the feedback information slavishly while a value of 0 could mean that instead another, possibly fixed precoder is used. The value zero would for example be used if the feedback information could not be correctly decoded at the transmitter. Obviously, all this assumes decoding errors in the feedback information can be detected, so the feedback information has to be coded accordingly, e.g. including a CRC. An alternative to a fixed precoder is to also signal a single “wideband” precoder. Several variants of precoder report verification schemes have been proposed and is also to be found in the IVD. Compared with explicitly signaling the frequency-selective precoding report in the forward link, verification approaches can substantially reduce the signaling overhead in the forward link.
The encoded bits, or modulated symbols, originating from the same block of information bits (transport block) is referred to as a codeword. This is also the terminology used in LTE to describe the output from a single HARQ process serving a particular transport block and comprises turbo encoding, rate matching, interleaving etc. The codeword is then modulated and distributed over the antennas. It may make sense to transmit data from several codewords at once, also known as multi-codeword transmission. The first (modulated) codeword may for instance be mapped to the first two antennas and the second codeword to the two remaining antennas in a four transmit antenna system. In the above context of precoding, the codewords are mapped to layers instead of directly to the antennas.
In the field of high rate multi-antenna transmission, one of the most important characteristics of the channel conditions is the so-called channel rank. Roughly speaking, the channel rank can vary from one up to the minimum number of transmit and receive antennas. Taking a 4×2 system as an example, i.e., a system with four transmit antennas and two receive antennas, the maximum channel rank is two. The channel rank varies in time as the fast fading alters the channel coefficients. Moreover, it determines how many layers, and ultimately also codewords, can be successfully transmitted simultaneously. Hence, if the channel rank is one at the instant of transmission of two codewords mapping to two separate layers, there is a strong likelihood that the two signals corresponding to the codewords will interfere so much that both of the codewords are erroneously detected at the receiver.
In conjunction with precoding, adapting the transmission to the channel rank involves using as many layers as the channel rank. In the simplest of cases, each layer would correspond to a particular antenna. But the number of codewords may differ from the number of layers, as in LTE. The issue then arises of how to map the codewords to the layers. Taking the current working assumption for the 4 transmit antenna case in LTE as an example, the maximum number of codewords is limited to two while up to four layers can be transmitted. A fixed rank dependent mapping according to FIG. 2 is used, although there have also been suggestions to include an additional mapping corresponding to mapping a single codeword to two layers.
A substantial amount of MIMO related information needs to be signaled in the forward link to support precoding with dynamic transmission rank adaptation. Taking LTE as an example, the physical downlink control channel (PDCCH) for the support of MIMO (Format 2) is currently proposed to contain the elements listed in Table 1. The fields in green contain MIMO related information. As seen, 16 bits are used for MIMO information, out of which 8 bits are related to precoding.
TABLE 1Contents of PDCCH format 2 in LTE.PDCCH Field#bitsResource allocationSystembandwidthdependentTransmitter power control2Transport block size (TBS) for codeword51: TBS1TBS for codeword 2: TBS25New data indicator (NDI) and redundancy3version (RV) for codeword 1NDI and RV for codeword 23HARQ process ID3PrecodingPrecoder matrix information (PMI)4infoPrecoder confirmation1Transmission rank: rank indicator (RI)2HARQ swap flag1Jointly Encoded Signaling of Precoder Related Information in the Forward Link
Another proposal on how to encode the TBS values and the precoder related information on the PDCCH in LTE for the 4 Tx case is as follows.
TBS values:
                The TBS values for the two codewords determine the interpretation of the precoder information bits        The TBS value pair (TBS1, TBS2) for the two codewords thus signals the following:        (TBS, 0): One codeword transmitted (with size TBS)        (TBS1, TBS2): Two codewords transmittedPrecoder information bits:        
Interpretation depends on (TBS1, TBS2)
Support of transmission rank override for precoder confirmation (support of frequency-selective precoding)
Override by using specified columns of all recommended precoders in the latest obtained precoder report conveyed from the receiver
RI, PMI and precoder confirmation jointly encoded as specified in Table 2 and Table 3.
Note: 1 codeword, RI=2 for 4 Tx corresponds to transmitting one codeword on layer 0 and 1.
Note: For 4 Tx in Table 2, the precoder column subset is implicitly know via the codeword to layer mappings in FIG. 2.
TABLE 2PDCCH for 4 Tx MIMO. The TBS entries point out the number of codewords and determine the interpretation of the precoder information, which is jointly encoded as shown below.Total#messagesforprecodinginfo#messagesMessage13416 + 1RI = 1: PMI = 0, 1, . . . , 15codeword:RI = 1: Precoder report confirmed,(TBS, 0)use the precoder indicated by thereported PMI index for eachprecoder report16 + 1RI = 2: PMI = 0, 1, . . . , 15RI = 2: Precoder report confirmed,use the precoder indicated by thereported PMI index for eachprecoder report25116 + 1R1 = 2: PMI = 0, 1, . . . , 15codewords:RI = 2: Precoder report confirmed,(TBS1,use the precoder indicated by theTBS2)reported PMI index for eachprecoder report16 + 1RI = 3: PMI = 0, 1, . . . , 15RI = 3: Precoder report confirmed,use the precoder indicated by thereported PMI index for eachprecoder reportProblems with Existing Solutions
For spatial multiplexing on 4 antenna ports, a single transport block can be mapped to two layers, depending on the transmission rank. For retransmissions of the transport block, it might be necessary to use a different transmission rank than what was used for the first transmission. This might occur when the channel rank for the retransmission instant has changed relative to the first transmission or if there is no data in the buffers for a second codeword. In any case, a possible consequence is that the same transport block needs to be retransmitted with a number of layers different from the number of layers used in the first or a previous transmission.
One of the open issues with the existing way of signaling TBS values in the forward link as exemplified by Table 1 and the approach related to Table 2 is how to perform retransmissions of transport blocks which need to be mapped to a different number of layers for succeeding retransmissions. Obviously, for a retransmission, the TBS value must be the same as for the previous transmissions since the number of information bits in the transport block does not change. But to save signaling overhead, the set of possible TBS values typically depend on the number of layers. This means it may not always be possible to retransmit with a different number of layers or there are substantial limitations which transport formats are supported for the given TBS value. Such problems are for example present in existing solutions, which use separate TBS tables for single-layer and dual-layer transport blocks, tables that only have partial overlap with respect to TBS values to support retransmissions with a varying number of layers.